1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexer (OFDM) system, and more particularly, to a fine fast Fourier transform (FFT) window position recovery apparatus of an OFDM system receiver.
2. Description of Related Art
In general, time synchronization should be performed accurately in order for a receiver to recover an OFDM signal transmitted in European digital broadcasts. Time synchronization includes FFT window position recovery, for a parallel process of a correct signal, and a sampling clock recovery, for controlling the sampling clock signal of an analog-to-digital converter (ADC), for sampling a portion in which the signal-to-noise ratio (SNR) is highest in a received signal.
FIG. 1 is a block diagram showing the structure of a conventional OFDM system receiving apparatus, which includes an ADC 110 for converting a received OFDM analog signal into a digital signal, a symbol start detector 120 for detecting the start of a symbol from the samples output by ADC 110, an FFT window controller 130 for generating an FFT window control signal from a symbol start signal output by symbol start detector 120, and an FFT 140 for fast Fourier transforming the data generated in ADC 110 based on the FFT window control signal generated by FFT window controller 130.
An OFDM signal symbol includes a guard interval, having G sample lengths, interposed between symbols in order to prevent interference between N effective data samples, which are the outputs of an inverse fast Fourier transform (IFFT), and the symbols when there are N FFTs. Namely, a guard interval copies the rear portion of an effective data section. A transmitter (not shown) transmits a symbol, including (G+N) samples, obtained by adding N complex values to G complex values output from an IFFT (not shown).
[Equation 1] ##EQU1##
Equation 1 represents the mth symbol formed of the complex value output from FFT 140. Here, m, k, N, and n represent a symbol number, a subcarrier number (index), the number of samples of the effective data and a sample time, respectively. In Equation 1, the first term, ##EQU2## represents a guard interval and the second term, ##EQU3## represents effective data.
As shown in FIG. 1, the received OFDM signal is converted into digital data by analog-digital converter (ADC) 110. In the sampled OFDM signal output from ADC 110, the start of a symbol is detected by symbol start detector 120 by detecting the position in which the cross-correlation value of a received signal is the highest. The second term is sequentially input to FFT 140 after the guard interval, which is the first term of Equation 1, is removed. An FFT window controller 130 designates the FFT window starting position for FFT 140 by using symbol start information from symbol start detector 120. Here, the first value output from a transmitting IFFT (not shown) must be input first to FFT 140. The Nth value output from the transmitting IFFT must be input Nth to FFT 140.
The first value output from the transmitting IFFT must be input to FFT 140 after searching for the start of a symbol in the receiver. Symbol start detector 120 detects the start of the symbol during an initial stage, as mentioned above. However, when the receiver is moving, symbol start detector 120 may not correctly estimate the start of a symbol due to receiver fading phenomena and the influence of its surroundings. Accordingly, the Nth value or the second value of the previous symbol is input first to FFT 140. Therefore, one value may be pushed or pulled and input to the FFT port. When the start of a symbol is not correctly estimated, the symbol cannot be correctly recovered. Accordingly, the performance of the system deteriorates.